This function converts its argument to DOUBLE, performs the computation in double-precision floating-point arithmetic, and
returns a DOUBLE as the result.

The population-based variance (s2) of numeric-expression (x) is computed according to the following formula:

s2 = (1/N) * SUM( xI - mean( x ) )2

This variance does not include rows where numeric-expression is NULL. It returns NULL for a group containing no rows.

Syntax 2 represents usage as a window function in a SELECT statement. As such, elements of window-spec can be specified either in the function syntax (inline), or in conjunction with a WINDOW clause in the SELECT statement.
See the window-spec definition for the WINDOW clause.

For more information about using window functions in SELECT statements, including working examples, see Window functions.

The ability to specify DISTINCT over an expression that is not a column reference comprises part of optional SQL language
feature F561, "Full value expressions". SQL Anywhere also supports SQL/2008 language feature F441, "Extended set function
support", which permits operands of aggregate functions to be arbitrary expressions possibly including outer references to
expressions in other query blocks that are not column references.

SQL Anywhere does not support optional SQL/2008 feature F442, "Mixed column references in set functions". SQL Anywhere does
not permit the arguments of an aggregate function to include both a column reference from the query block containing the VAR_POP
function, combined with an outer reference. For an example, see AVG function [Aggregate].